1. Field of the Invention
The present invention relates to a voltage regulator, and more particularly to a voltage regulator generating a second voltage having a desired potential difference from a first voltage.
2. Description of the Related Art
If a plurality of voltage systems are used in one apparatus, their voltage levels are sometimes adjusted so as to be in a relation of mutually having predetermined potential differences. For example, AC driving is performed in a liquid crystal panel in order to suppress the occurrence of deterioration and burnout of a liquid crystal, and the polarities of a video signal and a common electrode signal, which is the counter electrode signal of the video signal, are inverted every frame. In this case, the DC bias voltages of the video signal and the common electrode signal are set to have a predetermined potential difference from each other.
For example, Japanese Patent No. 3423193 and U.S. Pat. No. 6,281,871 B1 point out that, in the case of displaying a fully colored image by a video signal, respective R, G, and B signals are provided to a liquid crystal panel after they are converted into alternating current signals. The publications describes that, if the center potential of the respective R, G, and B alternating current signals differs from that of the opposed electrode, then problems occur, such as burnout, difference in white balance, or degradation of contrast. Moreover, the publications describe that, in order to make the center voltage uniform among the R, G, and B signals, the AC signal applied to the liquid crystal panel is converted to a DC voltage by a smoothing circuit, and is compared with a reference voltage, which is the center point of the AC signal, in a comparator, and that the comparator output is fed back to a bias current of a differential output amplifier so that the center potential of the AC signal is made to conform to the reference voltage.
Japanese Patent Laid-Open Publication No. Sho 61-249094 describes that a video signal having a polarity inverted every field is applied to the Y electrode of one pixel of a matrix type liquid crystal display apparatus, and that a common electrode signal having a voltage value inverted every field is applied to a common electrode, and further that the relation between the video signal and the common voltage becomes incorrect owing to the dispersion of interelectrode capacitance and storage capacitors. The patent publication discloses that a video signal whose polarity is inverted by a polarity inverting circuit is taken out from the emitter follower of a transistor, and that the emitter follower is connected to a current source composed of a transistor and a variable resistor with a resistor connected between them to change the current of the current source with the variable resistor in order to change the voltage level of the resistor so that the DC level of the video signal having the inverted polarity may be changed.
As described in the examples above, in a liquid crystal display apparatus, the DC potential difference between a video signal and a common electrode signal is determined in accordance with the specification of the liquid crystal display apparatus, and the DC potential difference is adjusted in conformity with the specification. For the voltage adjustment, the following methods can be used: a method of performing feedback to the bias current of the differential output amplifier based on the comparison of the reference voltage and the center voltage of an AC signal, which method is described in the U.S. Pat. No. 6,281,871 B1, a method of changing a current to change the voltage level between both ends of a resistor, which method is described in the Japanese Patent Laid-Open Publication No. Sho 61-249094, and the like.
However, these related art technologies collectively adjust the contents of two steps at one time. Moreover, because the related art adjusts the contents by shifting them from the reference state, an error of the adjustment becomes larger as the adjustment range from the reference becomes larger. For example, in the case of the method disclosed in the U.S. Pat. No. 6,281,871 B1, when a desired potential difference is large, the center voltage of the AC signal, i.e. the value of a DC level, becomes larger, and consequently the value of the reference voltage becomes larger to increase a setting error by that much. In the case of the method disclosed in the Japanese Patent Laid-Open Publication No. Sho 61-249094, the voltage level between both ends of the resistor is set to become larger as the desired potential difference becomes larger, and consequently the setting error becomes larger by that much.
As described above, with the methods of the related art, errors become larger as the extent of voltage adjustment becomes larger, and it is sometimes difficult to obtain a correct potential difference.